Conventional anti-aircraft guns such as the internationally used Bofors L-70 40 mm automatic gun use spin-stabilized projectiles whose flight path is not controllable after the projectile leaves the gun barrel. Because of aerodynamic drag and weight considerations, the range of these conventional projectiles is only on the order of 4 km. Because of the lack of control over their flight path, a substantial average number of rounds is required per hit, and the projectiles are of limited effectiveness against jinking targets.
A need therefore exists for a guidance system to control the flight path of a spinning projectile which can be used in existing anti-aircraft guns such as the Bofors L-70. Such a guidance system would dramatically reduce the average number of rounds per hit (thereby greatly reducing the problem of supply logistics), and would also make the projectile highly effective against jinking targets.
The need for compatability, in size and shape, with conventional 40 mm ammunition imposes a number of restraints upon the guidance system, as does the need for aerodynamic optimization to maximize the projectile's range. Specifically, the size and shape of that portion of the projectile which protrudes from the cartridge casing cannot be altered, and the guidance system must therefore be placed inside the casing at the aft end of the projectile. This in turn requires the guidance system to withstand not only longitudinal acceleration forces exceeding 50,000 g, but also the tremendous breech pressures which develop within the casing when the round is fired. Consequently, the exposed portions of the guidance system must have a structural integrity which prohibits the use of hinged fins or complex actuating mechanisms.
The projectile has, of necessity, a spin imparted to it by the rifling of the gun barrel. This spin can be reduced but not eliminated. Consequently, prior art devices had to rely on complex, rapid-acting guidance systems to change the attitude of flight control fins in synchronism with the spin of the projectile to achieve a consistent flight path. Such systems required highly sophisticated electronics and large amounts of battery power.